Method and Apparatus for a Wireless Communication Receiver using Serial Localization with Indecision

ABSTRACT

In one aspect, the present disclosure provides an advantageous generalization of a SLIC receiver structure, by allowing multiple solutions to survive at any one or more of the serial stages included in the SLIC receiver. Where any given stage produces multiple solutions, the next stage of the SLIC receiver processes those multiple input solutions to produce multiple output solutions. Consequently, the contemplated SLIC receiver effectively builds a tree structure with as many levels as there are stages in the SLIC receiver. Such operations allow the SLIC receiver to form two or more solution threads spanning the stages, and to make an improved overall demodulation decision for a received symbol vector by selecting the best solution thread. Further, the additional complexity of allowing multiple survivor solutions at one or more SLIC stages is controlled in one or more embodiments, using survivor pruning for example.

TECHNICAL FIELD

The present invention generally relates to wireless communications, andparticularly relates to wireless communication apparatuses using SerialLocalization with Indecision (“SLIC”) based demodulation.

BACKGROUND

The acronym “SLIC” refers to one approach to addressing the complexitiesassociated with demodulating high-data rate signals, with theirgenerally higher-order modulation constellations. SLIC, or “SerialLocalization with Indecision,” uses a multi-stage receiver structure,where the individual stages each operate on a simplified (reduced-ordermodulation) constellation rather than the full-order modulationconstellation(s) associated with the symbols in a received symbolvector.

As noted, SLIC receivers are multi-stage, serial receivers, where eachstage includes a joint processing unit. While not limited toMultiple-Input-Multiple-Output (MIMO) scenarios, SLIC represents aparticularly advantageous type of reduced-complexity demodulator forMIMO-based receivers. In a MIMO system with N transmit streams, thejoint processing unit in a given SLIC stage performs a full search overall candidate N-tuples of centroids. Each centroid represents a subsetof symbols from the actual modulation constellation(s) used fortransmitting the symbol vector.

The use of centroids creates a residual signal, which belongs to afinite set. The residual signal is effectively modeled as a coloredGaussian noise, having a covariance that is derived from the channelcoefficients. Further, the search metric in the joint processingfunction is the Euclidean metric, with a total covariance that includesthat of the residual signal. In a “conventional” SLIC formulation, thejoint processing unit at a given SLIC stage produces a unique solution,which is accounted for in the input signals propagated from that givenstage to the next stage. Thus at a stage i, the previous (i−1) solutionsare unique, and accounted for in its input. With this in mind, it willbe appreciated that a key feature of SLIC is the use of overlappingsignal subsets. This overlap mitigates the effect of incorrect decisionsin early stages on later stages.

Various examples of SLIC receiver structures and associated SLICprocessing are known. For example, the interested reader may refer toany one or more of the following published U.S. patent applications:US20110051795A1, US20110051796A1, US20110051851A1, US20110051852A1,US20110051853A1, US20110096873A1, US20110103528A1, US20110243283A1,US20110255638A1, US20110261872A1, and US20120027139A1, all of which areincorporated by reference herein. However, in the interest of presentinghelpful background details within this disclosure, consider aconventional SLIC receiver, including a joint processing unit (i.e., ajoint demodulator or detector that jointly detects symbols) in eachserial stage.

For the MIMO scenario and joint detector, one may consider the N×N case,where there are N transmit antennas and N receive antennas—e.g., a 2×2MIMO scenarios. For a non-dispersive channel, the received signal is

r=Hc+n.  (1)

Here r, c and n are N×1 vectors, and H is a N×N matrix. The componentsof H are independent and Rayleigh faded, and n is white Gaussian noisewith covariance R. Further, for this example, assume that all N signalsare from the same constellation Q of size q, and all N signals aretransmitted with the same power. The effective constellation for c is ofsize q^(N).

The optimal receiver in this scenario conducts a full search over allq^(N) candidates c=( c ₁, . . . , c _(N))^(T) in Q^(N) for one thatminimizes the metric

D( c|r,R)=(r−H c )^(H) R ⁻¹(r−H c ),  (2)

where subscript T indicates the transpose, and subscript H indicates theHermitian, or conjugate transpose. The best candidate is denoted ĉ.

The general idea of SLIC is to represent the symbols in a transmittedsymbol vector by a series of approximations. In an L-stage SLIC, thesymbol vector is effectively represented as

c=c ^([1]) + . . . +c ^([L]),  (3)

where stage i detects component c^([i]), using an effective alphabetQ^([i] derived from the true alphabet Q.)

While a good general formulation of SLIC-based receiver processingappears in US20110051851A1, as listed above, a few key characteristicsare worth expanding herein. To with, in the first stage of a SLICreceiver, the true alphabet Q used for transmission of the symbol vectoris approximated by a set of centroids Q^([i]), of size q^([i])<q. Notethat Q is also referred to herein as the “actual modulationconstellation” for the received symbols of interest.

Each centroid represents a subset of Q. Moreover, the subsets have threeproperties: (1) the subsets overlap; (2) their union is equal to Q; and(3) all the subsets are shifted versions of the same set O^([1]) of sizeo^([1]), with centroid equal to zero (“0”). The centroid overlapproperty is a key ingredient, as it enables the indecision feature ofSLIC, which in turn boosts demodulation performance.

If L>2, in the second stage O[1] plays the role of Q. That is, O[1] isapproximated by a set of centroids Q^([2]), of size q^([2]), with thesame three properties listed above and based on a set O^([2]) of sizeo^([2]), with centroid 0. We proceed similarly for all stages except thelast.

For the last stage L, there is no more approximation, thusQ^([L])=O^([L−)1], and O^([L]) is empty. The outcome consists of thesets Q^([1]), . . . , Q^([L]), which serve as the effectiveconstellations for the L stages of the SLIC receiver.

Consider, for example, the 16-QAM constellation. An example SLICprocessing approach “covers” the 16-QAM constellation with fouroverlapping subsets. This approach is shown in FIG. 1. Each subset is ashifted version of 9-QAM, so that O^([1]) is 9-QAM. The four respectivecentroids correspond to 4-QAM or QPSK, so Q^([1]) is 4-QAM. In atwo-stage SLIC receiver, Q^([1]) serves as the constellation of stage 1,and Q^([2])=O^([1]) as the constellation of stage 2. One may refer tothis subset design and to the corresponding SLIC receiver as (4,9).

One may also choose to cover the set O^([1]) itself with fouroverlapping subsets, such as shown in FIG. 2. Each subset is a shiftedversion of 4-QAM, so that O^([2]) is 4-QAM, as is shown in FIG. 3. Thefour respective centroids are shown as circles, and also correspond to4-QAM, so one sets Q^([2]) to be 4-QAM. In a 3-stage SLIC, Q^([1]),Q^([2]) and Q^([3])=O^([2]) serve as the constellations of stages 1, 2and 3, respectively. One may refer to this subset design and to thecorresponding SLIC receiver as (4,4,4).

Note that these subset designs are not unique. For instance, the paperby A. Khayrallah, “SLIC—A Low Complexity Demodulator for MIMO,” Proc.IEEE VTC-Fall, 2011, presents a number of subset designs for 16, 64 and256-QAM.

Marrying the above details to an example SLIC receiver, FIG. 4 depictsstage i of a known SLIC receiver architecture. The stage input signal isthe modified received signal r^([i−1]) from prior, i.e., the stage(i−1). Of course, for the first stage, the stage input signal is the(starting) received signal of interest. Stage i assumes that thecomponents c^([1]), . . . , c^([i−1]) have been determined in earlierstages, and focuses on the demodulation of c^([i]). The (stage) residualsignal

b ^([i])=(c ^([i+1]) + . . . +c ^([L]))  (4)

belongs to the set O^([i]). Stage i models Hb^([i]), the residual signalfiltered by the channel, as a colored Gaussian noise, with covariance

ε^([i]) HH ^(H)  (5)

where ε^([i]) is the average energy of the set O^([i]).

One problematic aspect here is that useful knowledge about O^([i]) isnot being exploited, which is among a number of advantages attendingparticular embodiments of the present invention. In any case, note thatε^([i]) decreases as the stage index i grows because there is less andless of the received signal being demodulated that is unaccounted for inthe demodulation process as processing progresses through thesuccessive, serial stages of the stages of the SLIC receiver.

The total noise has covariance

R ^([i])=ε^([i]) HH ^(H) +R  (6)

The joint processing unit—i.e., the joint detection unit in eachstage—conducts a full search over constellation Q^([i]). It searchesover all (q^([i]))^(N) candidates c ^([i]) in (Q^([i]))^(N) for the onethat minimizes the metric

D( c ^([i]) |r ^([i−1]) ,R ^([i]))=(r^([i−1]) −H c ^([i]))^(H)(R^([i]))⁻¹(r ^([i−1]) −H c ^([i]))  (7)

which is modified from (2) by replacing R with R_([i]), and the receivedsignal r with the modified received signal r^([i−1]).

Assuming that stage i is not the last stage, the stage generates asre-modulated signal as {circumflex over (r)}′^([i])=Hĉ^([i]) and thenforms a modified received signal for propagation to the next stage as astage output signal. The modified received signal is expressed as

r ^([i]) =r ^([i−1) ]−Hĉ ^([i]),  (8)

which is fed to the next stage, denoted as stage (i+1). So, each stage iexcept for the first stage, receives as its stage input signal themodified received signal r^([i−1]) provided as the stage output signalfrom the prior stage (i−1). Further, each stage i except for the laststage, outputs its own stage-specific modified received signal r^([i]),as the stage output signal to be provided to the next stage (i+1) as ther^([i−1]) stage input signal at that next stage.

As for the first stage where i=1, its stage input signal is the originalreceived signal r. Further, in a non-limiting example of the SLICarchitecture, the last stage where i=L, the constellation Q^([L]) is asubset of Q. Because there is no next stage after the L-th stage, thereis no need for a re-modulation block in the L-th stage.

The overall symbol decision made by the SLIC receiver for the receivedsymbol vector is found by adding all the intermediate stage decisionvectors ĉ^([i]) output from each of the SLIC receiver stages. As such,the overall symbol decision is expressed as

ĉ=ĉ ^([1]) + . . . +ĉ ^([L]).  (9)

The serial arrangement of successive SLIC stages and the overall SLICstructure are shown in FIG. 5.

Note that stage i is affected by the sequence of (i−1) unique previousdecisions (ĉ^([1]), . . . , ĉ^([i−1])). This can be seen by writing theinput r^([i]) in (8) explicitly as

r ^([i]) =r−H(ĉ ^([1]) + . . . +ĉ ^([i−1]))=r−Hŝ ^([i−])  (10)

where the decision state of the SLIC receiver at the output of stage i−1is

ŝ ^([i−1]) =ĉ ^([1]) + . . . +ĉ ^([i−1]).  (11)

Thus,ŝ^([i−1] summarizes the impact of all preceding stage decisions, i.e., (ĉ)^([1], . . . , ĉ) ^([i−1]). One may therefore think of ŝ^([i−1]) as the“state” of the SLIC receiver at the end of processing for stage i−1.

Because of the subset overlap feature, it is possible that a differentsequence of candidates ( c ^([1]), . . . , c ^([i−1])) also satisfiesthe same value

( c ^([1]) , . . . , c ^([i−1]))=ŝ ^([i−1]),  (12)

meaning that ( c ^([1]), . . . , c ^([i−1])) would have the same impactas (ĉ^([1]), . . . , ĉ^([i−1])) on stage i. Furthermore, the effect of (c ^([1]), . . . , c ^([i−1])) is the same as (ĉ^([1]), . . . ,ĉ^([i−1])) on the eventual solution, i.e., the overall symbol decision ĉin (9). As such, they are interchangeable for purposes of thisexplanation, thus confirming the notion of ŝ^([i−1]) as the state ofSLIC receiver. Notably, at the input to each stage i, there is only onesurviving value of the state ŝ^([i−1]) provided by the previous stagei−1.

SUMMARY OF THE INVENTION

In particular embodiments, the present invention advantageouslygeneralizes the SLIC receiver structure, by allowing multiple solutionsto survive at any one or more of the serial stages included in the SLICreceiver. Where any given stage produces multiple solutions, the nextstage of SLIC receiver processes those multiple input solutions toproduce multiple output solutions. Consequently, the contemplated SLICreceiver effectively builds a tree structure with as many levels asthere are stages in the SLIC receiver. Such operations allow the SLICreceiver to form two or more solution threads spanning the stages, andto make an improved overall demodulation decision for a received symbolvector by selecting the best solution thread. Further, the additionalcomplexity of allowing multiple survivor solutions at one or more SLICstages is controlled in one or more embodiments, using survivor pruningfor example.

In one or more embodiments, a wireless communication apparatus includesa SLIC receiver that is configured to demodulate a received symbolvector conveyed in a received signal. The SLIC receiver includes a setof two or more serial stages. Each stage is configured to output atleast one stage decision vector representing a stage-specificdemodulation solution for the received symbol vector according to astage-specific modulation constellation. More particularly, at least onestage is configured to output more than one stage decision vector, thusproviding two or more surviving solutions at that stage.

Correspondingly, each stage except for the last stage is configured topropagate to a next stage one or more modified received signals. Eachmodified received signal comprises, if the stage is the first stage, thereceived signal as modified by the stage to account for a particular oneof the one or more stage decision vectors output by the first stage.Otherwise, if the stage is not the first stage, each stage output signalcomprises one of the one or more modified received signals propagatedfrom the prior stage, as modified by the (current) stage to account fora particular one of the one or more stage decision vectors output by the(current) stage, for the corresponding modified received signal providedto the (current) stage.

Because at least one stage of the SLIC receiver produces two stagedecision vectors, the SLIC receiver can be understood as producing atleast two solution threads. Each solution thread comprises a differentcombination of the stage decision vectors taken across the set of two ormore serial stages. That is, each solution thread is a differentcombination of stage decision vectors taken across all stages of theSLIC receiver, with one such stage decision vector from each stage.

Still further, the SLIC receiver includes a decision processor that isconfigured to generate two or more candidate symbol vectors for thereceived symbol vector, from respective ones of two or more of thesolution threads. Each such candidate symbol vector represents anoverall demodulation solution for the received symbol vector and thedecision processor is configured to identify a best one of the candidatesymbol vectors and to output the best candidate symbol vector as theoverall demodulation solution for the received symbol vector.

In one or more other embodiments, a method of demodulating a receivedsymbol vector conveyed in a received signal includes outputting at leastone stage decision vector from each stage in a series of two or morestages of a SLIC receiver. More particularly, the method includesoutputting at least two stage decision vectors (two survivor solutions)from at least one of the stages of the SLIC receiver. Each stagedecision vector represents a stage-specific demodulation solution forthe received symbol vector according to a stage-specific modulationconstellation.

The method further includes each but the last stage propagating to thenext stage a modified received signal for each of the one or more stagedecisions by the stage. Here, each modified received signal output by agiven stage is formed as a function of a corresponding one of the stageinput signals received by the stage and corresponding one of the one ormore stage decision vectors generated by the stage for thatcorresponding stage input signal. For the first stage there is only onestage input signal, namely, the received signal conveying the receivedsymbol vector.

Still further, the method includes forming two or more solution threadsas different combinations of stage decision vectors. Each differentcombination being formed by taking one stage decision vector from eachof the stages in the SLIC receiver. Correspondingly, the method includesgenerating a candidate symbol vector for at least two of the two or moresolution threads, evaluating the candidate symbol vectors to identify abest one of the candidate symbol vectors, and outputting the bestcandidate symbol vector as an overall demodulation solution for thereceived symbol vector.

Of course, the present invention is not limited to the above featuresand advantages. Indeed, those skilled in the art will recognizeadditional features and advantages upon reading the following detaileddescription, and upon viewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-3 are example diagrams of known modulation constellations andsubset mappings used for SLIC receiver processing.

FIGS. 4 and 5 are example diagrams of known arrangements forconventional SLIC receivers.

FIG. 6 is a block diagram of one embodiment of a wireless communicationapparatus that is configured as a transceiver for operation in awireless communication network.

FIG. 7 is a diagram of a typical received symbol vector, such as mightbe conveyed on multiple streams in a MIMO signal, in a multi-codedsignal, etc.

FIG. 8 is a block diagram of one embodiment of a SLIC receiverimplemented in the wireless communication apparatus of FIG. 6, forexample.

FIG. 9 is a block diagram further illustrating the relationship betweendifferent decision threads and stage-specific decisions, for a SLICreceiver that produces multiple stage decision vectors at one or more ofits stages.

FIG. 10 is a block diagram of another embodiment of a SLIC receiver,including particular details for a three-stage example configuration.

FIG. 11 is a diagram of example solution threads formed by a SLICreceiver as contemplated herein.

FIG. 12 is a block diagram of one embodiment of a decision processor,which is configured to evaluate multiple solution threads, as producedin the SLIC receiver of FIG. 8 or FIG. 10, for example.

FIG. 13 is a logic flow diagram of one embodiment of a method ofdemodulating a received symbol vector using SLIC-based demodulation thatinvolves generating and using multiple solution threads.

DETAILED DESCRIPTION

FIG. 6 illustrates a wireless communication apparatus 10 that receives a“composite” signal 12 that includes or otherwise conveys a symbol vector14 of N symbols 16. As a non-limiting example, the composite signal 12carries multiple data streams or other multiple components, each ofwhich conveys one or more symbols 16 in one or more repeating symboltransmission times. As a particular non-limiting example, the compositesignal 12 is a MIMO transmission signal that includes multiple datastreams, each data stream conveying information symbols transmittedaccording to, e.g., 16-QAM and/or 64-QAM symbol constellations.

The wireless communication apparatus 10 is, in an example embodiment,configured for use in a wireless communication network—e.g., thewireless communication apparatus 10 comprises a cellular telephone orother wireless communication apparatus configured to receive downlink(DL) signals from a wireless communication network and to send uplink(UL) signals to that network. Correspondingly, for operation as awireless transceiver, the wireless communication apparatus 10 includes anumber of antennas 22 for receiving the composite signal 12, and furtherincludes receiver processing circuits 24, transmitter processingcircuits 26, and may include additional circuitry 28.

The nature and extent of the additional circuitry 28 depends on theintended use and particular implementation of the wireless communicationapparatus 10, and may include “system” type microprocessors, memory, anduser interface components, such as display screens, keypads,touchscreens, etc. Of course, the present invention is not limited tohandsets or other remote terminal devices, and may be incorporated intoor implemented by other types of receivers, such as those used in theradio network nodes of a wireless communication network.

With the above in mind, FIG. 7 depicts an example symbol vector 14 ofsymbols 16, e.g., such as may exist in the buffered data available tothe receiver processing circuits 24 for one or more symbol times of thereceived composite signal 12. Correspondingly, according to the furtherexample details of FIG. 8, the receiver (“RX”) processing circuits 24 inone embodiment include at least a front-end (RF) circuit 30 thatgenerates a received signal 32 that corresponds to the antenna-receivedcomposite signal 12. The received signal 32 is also denoted as thereceived signal “r” in the subsequent mathematical formulae and it willbe understood that the received signal 32 conveys the received symbolvector 14.

The RX processing circuits further include a SLIC receiver 34 which ingeneral has two or more stages 36, and which in the example of FIG. 8has stages 36-1, 36-2, . . . , 36-L. The stages 36 also may be referredto as “stage 1,” “stage 2,” and so on. Each stage 36 includes one ormore stage inputs 38, for receiving one or more stage input signals 40.For the first stage 36-1, there is one stage input signal 40, namely,the received signal 32. The number of stage input signals 40 received byany one of the subsequent stages 36 depends on the number of stagedecision vectors 42 that are output from/used by the immediately priorstage 36. Notably, in contrast to conventional SLIC receivers asdiscussed in the Background of this document, one or more stages 36 ofthe SLIC receiver 34 output multiple stage decision vectors 42, e.g.,more than one stage decision vector ĉ^([i]), of the type described inthe Background of this document.

In simple terms, in one embodiment contemplated herein, any given stage36 of the SLIC receiver 34 produces K stage decision vectors 42 for eachstage input signal 40 applied to the stage 36. Letting R denote thenumber of stage input signals 40 received by a given stage 36, then ajoint detection processor 44—e.g., a joint demodulationcircuit—generates K stage decision vectors 42 for each one of the Rstage input signals 40, yielding K×R stage decision vectors 42. LettingR=2 and K=2 as non-limiting examples for a given stage 36, the jointdetection processor 44 of the given stage 36 would generate 2×2=4 stagedecision vectors 42.

Optionally, however, each stage 36 includes a pruning circuit 46 that isconfigured to “prune” the actual number of stage decision vectors 42that are output from and used by the stage 36 to some number M. In thetrivial case, M=K×R, meaning that the raw number of stage decisionvectors 42 generated by the joint detection processor 44 is the numberoutput from and used by the given stage 36. Omitting or disabling thepruning circuit 46, or setting M to the value of K×R, achieves thetrivial case.

In a more meaningful example, M is some number less than K×R, meaningthat the number of stage decision vectors 42 that are output from andused by the stage 36 is less than the raw number generated by the jointdetection processor 44. For example, assume that K×R=8, and that M isset to a value of four. As such, the stage 36 outputs and uses fourstage decision vectors 42, rather than eight. For example, pruning mayselect the “best” four stage decision vectors 42. As an example, the“best” stage decision vectors 42 are identified in terms of theirprobabilities or some metric related thereto, for example.

In relation to the above explanation, it will be appreciated that thejoint detection processor 44 of each stage 36 may include a singledemodulation unit that performs a joint demodulation for each stageinput signal 40. That is, when R>1, meaning that there is more than onestage input signal 40 to be demodulated by the stage 36, the jointdetection processor 44 can use its demodulation unit to jointlydemodulate the multiple stage input signals 40 one at a time.Alternatively, as is suggested in the illustration, the joint detectionprocessor 44 may be equipped, at least functionally, with multipleinstances of a joint demodulation unit, where each instance jointlydemodulates a different one of the R stage input signals 40.

As a further note regarding the above-explained operations, the phrases“output from” and “used by” warrant further explanation. First, eachgiven stage 36 outputs M stage decision vectors 42. The value of M isfixed as a matter of design in one embodiment; in another embodiment itis set as a configuration value; and in yet another embodiment it isdynamically varied as an operating parameter. As a further extension toany one of these embodiments, the value of M is not necessarily the samefor each stage 36.

In any case, for each stage 36, the value of M represents the number ofstage decision vectors 42 that are output from the stage 36 forevaluation by a decision processor 48 that is included in control andprocessing circuits 50. As will be explained later herein, the decisionprocessor 48 forms “solution threads” 52—e.g., 52-1, . . . , 52-T, whereT is >1—from different combinations of stage decision vectors 42 takenacross all stages 36. That is, each solution thread 52 represents adifferent combination of stage decision vectors 42 taken across allstages 36 and formed by selecting a particular one of the M stagedecision vectors 42 output from each stage 36.

The decision processor 48 evaluates these solution threads 52 todetermine a final, overall demodulation solution for the received symbolvector 14, denoted in the diagram as the “best” candidate symbol vector54. An associated decoding circuit 56 decodes the (soft) bit informationcomprising the best candidate symbol vector 54 to obtain recoveredtransmit information 58.

As for the number M of stage decision vectors 42 output from each stage36, without pruning, the value of M at each stage 36 equals K×R. Inother words, without pruning, the number of stage decision vectors 42output from the given stage 36 equals the raw number (K×R) of stagedecision vectors 42 that is generated by the joint detection processor44 of the stage 36, with the joint detection processor 44 generating Ksuch stage decision vectors 42 for each one of the R stage input signals40. With pruning, K×R>M, and it will be understood that pruning offersan advantageous way to limit “fan out” as processing moves through thesuccessive serial stages 36, by selecting at each given stage 36 onlythe M best stage decision vectors 42.

Fan-out also may be controlled via the value of K. As with the value ofM, the value of K may be fixed or configurable, or dynamically varied asan operating parameter, and may or may not be the same value across allstages 36 of the SLIC receiver 34. Indeed, it is an aspect of one ormore embodiments herein to use different values of K and/or M fordifferent stages 36. The control and processing circuits 50 are, forexample, configured to provide each stage 36 with one or more signal(s)indicating the value(s) of K and/or M to be used by the stage 36. In onesuch embodiment, the control and processing circuits 50 include apruning processor 60 that controls the operation of the pruning circuits46 in the stages 36, e.g., enabling/disabling pruning and/or providingthe value(s) of M to be used by each such pruning circuit 46.

In any case, M stage decision vectors 42 are “output from” each stage36. At a minimum, M=1, and for at least one stage 36 of the SLICreceiver 34, M>1. Correspondingly, each stage 36 also “uses” the M stagedecision vectors 42 to produce M stage output signals 62 that arepropagated to the next stage 36, serving as the R stage input signals 40for that next stage 36. That is, the value of Mat stage 36-(i−1)determines the number of stage output signals 62 that are propagated tothe next stage 36-i, and hence determines the number R of stage inputsignals 40 received by stage 36-i.

Each stage output signal 62 that is output by a given stage 36 is amodified version of a corresponding one of the R stage input signals 40that is input to that stage 36. More particularly, each stage outputsignal 62 is a function of a particular one of the M stage decisionvectors 42 output from the stage 36, which in turn represents astage-specific joint symbol decision for the received symbol vector 14determined from a particular one of the R stage input signals 40.

A re-modulation circuit 64 included in each but the last stage 36-Lprovides the M stage output signals 62 from corresponding stage outputs66. Note that the re-modulation circuit 64 in each stage 36 alsoreceives the R stage input signals 40 input to the stage 36. Using atemporary notation for this explanation, assume that a given one of theM stage decision vectors 42 output from a given stage 36-i is denoted asĉ^([i])(m|r^([i−1])(r)), where “m” indicates the m-th one of the stagedecision vectors 42 and where r^([i−1])(r) denotes the r-th one of thestage input signals 40 from which ĉ^([i])(m|r^([i−1])(r)) was derived.Furthermore, for each output ĉ^([i])(m|r^([i−1])(r)), there is anassociated corresponding metric, which can be written explicitly as

D(ĉ ^([i])(m|r ^([i−1])(r))|r ^([i−1])(r),R ^([i]),  (13)

Continuing with the temporary notation, the corresponding re-modulatedsignal may be denoted as {circumflex over (r)}′^([i])(m|r^([i−1])(r)),which is generated by the re-modulation circuit 64 as

{circumflex over (r)}′ ^([i])(m|r ^([i−1])(r)=Hĉ ^([i])(m|r^([i−1])(r)),  (14)

where H as before denotes the channel estimates. Then, the correspondingstage output signal 62, denoted here as 62-m, is formed by subtractingthe re-modulated signal {circumflex over (r)}′^([i])(m|r^([i−1])(r))from the particular stage input signal r^([i−1])(r). That is, each oneof the M stage output signals 62 is formed as

r ^([i])(m)=r ^([i−1])(r)−Hĉ ^([i])(m|r ^([i−1]() r)),  (15)

which is fed to the next stage 36-(i+1). (The above temporary notationis not used going forward.)

One aspect of the present invention therefore can be understood as ageneralization of the SLIC structure, based on allowing multiplesolutions to survive, in the form of multiple values of the stateŝ^([i−1]). The SLIC receiver 34 provides this feature by allowing thejoint detection processor 44 of a given stage 36 to produce multiplesolutions—i.e., to generate multiple stage decision vectors 42. At thenext stage 36, those multiple solutions are fed to parallel instances,for example, of the demodulation units included within the jointdetection processor 44, each with a different input corresponding to adifferent solution.

As a result, the SLIC receiver 34 effectively creates a tree structurewith as many levels as there are stages 36. To keep the total complexitymanageable, the SLIC receiver 34 may perform pruning of the solutiontree, by comparing the multiple solutions generated at each stage, andkeeping a fixed number of best solutions, in the manner of theM-algorithm.

One may thus generalize the joint detection processor 44 at each stage36 by enabling its full search using the metricD(ĉ^([i])|r^([i−1]),R^([1])) to output the best K solutions for ĉ^([i])in (Q^([i]))^(N), instead of just the best one. Furthermore, to each ofthose K solutions out of a demodulation unit, there is an associatedcorresponding metric. If pruning is applied at the present stage i, thenthe totality of solutions for ĉ^([i]) from all the demodulation units atstage i are compared on the basis of their associated metrics. The Mbest solutions for ĉ^([i]) are kept, and the other solutions arediscarded.

The above generalization changes how the SLIC 34 operates in comparisonto known SLIC receivers, and imposes a tree structure. Starting fromstage 1 (36-1), where the stage input signal 40 is r, there are K outputvalues for ĉ^([1]). For clarity, from here on this disclosure usesexplicit indexing, showing the indices of the solutions from all theprevious stages. In particular, the K output values from the first stage36-1 are denoted as ĉ^([1])(k₁) for k₁=1, . . . , K.

To accommodate these K values, one may generalize stage 2 (stage 36-2)accordingly, with K instances of the stage 2 joint detection processor44 operating in parallel—e.g., parallel joint demodulation units, eachoperating on one of the solutions from stage 1. For clarity, theparallel instances are depicted as parallel demodulation units in, e.g.,FIG. 8.

Each of the K stage 2 demodulation units corresponds to a differentvalue of the state ŝ^([1])(k₁)=ĉ^([1])(k₁), resulting in a differentinput value r^([1])(k₁)=r−Hĉ^([1])(k₁). That is, stage 2 unit k₁operates on the stage output signal 40 denoted as r^([1])(k₁) from stage1, and produces a corresponding K output value for ĉ^([2]), which isexplicitly denoted ĉ^([2])(k₁,k₂), for k₂=1, . . . , K. Overall, thereare K² values for ĉ^([2]) out of stage 2, assuming that stage 1propagates K stage output signal 62 to stage 2 and that stage 2generates the same number K stage decision vectors 42 for each one ofthose K stage output signals 62 received by stage 2 as stage inputsignals 40—one will recognize from the earlier notation of FIG. 8 thatwhen the number M of stage output signals 62 propagated from a givenprior stage 36 equals the number K of stage decision vectors 42generated by that given prior stage 36, then the number R of stage inputsignals 40 received by the next stage 36 will equal the value of K ofthe given prior stage 36. In any case, for stage 2 in this example, eachstage decision vector 42 at stage 2—denoted asĉ^([2])(k₁,k₂)—corresponds to a value of the stateŝ^([2])(k₁,k₂)=ĉ^([1])(k₁)+ĉ^([2])(k₁,k₂).

Because of the overlap of SLIC subsets, some of the state valuesŝ^([2])(k₁, k₂) may coincide. While pruning may be applied, for now onemay assume that all solutions are kept at each stage 36, meaning thatM=K at each stage 36, and that R at one stage 36 equals K at the priorstage 36. Thus, continuing in the same vein, there are K^(i−1) values ofthe state ŝ^([i−1]) after stage i−1, denoted ŝ^([i−1])(k₁, . . . ,k_(i−1)). There are also K^(i−1) values of the modified received value

$\begin{matrix}\begin{matrix}{{r^{\lbrack{i - 1}\rbrack}\left( {k_{1},\ldots \mspace{14mu},k_{i - 1}} \right)} = {{r^{\lbrack{i - 2}\rbrack}\left( {k_{1},\ldots \mspace{14mu},k_{i - 2}} \right)} -}} \\{{H{{\hat{c}}^{\lbrack{i - 1}\rbrack}\left( {k_{1},\ldots \mspace{14mu},k_{i - 1}} \right)}}} \\{= {r - {H{{\hat{s}}^{\lbrack{i - 1}\rbrack}\left( {k_{1},\ldots \mspace{14mu},k_{i - 1}} \right)}}}}\end{matrix} & (16)\end{matrix}$

To accommodate these values, each stage i provides a correspondingnumber of parallel demodulation units, such as shown in FIG. 8. Stagedemodulation unit (k₁, k_(i−1)), accepts r^([i−1])(k₁, . . . , k_(i−1)),and produces K values of ĉ^([i]), denoted ĉ^([i])(k₁, . . . , k_(i)),for a total of K^(i) values. After Stage i, at the output of unit (k₁, .. . , k_(i−1)), the new state can be written as

ŝ ^([i])(k ₁ , . . . ,k _(i))=ŝ ^([i−1])(k ₁ , . . . ,k _(i−1))+ĉ^([i])(k ₁ , . . . ,k _(i)).  (17)

Note that the indices (k₁, . . . , k_(i−1)) tell exactly how the SLICreceiver 34 got to that point. Hence it is helpful to view the wholeprocess as a K-ary tree structure with L levels, where L is the numberof stages 36. FIG. 9 illustrates an un-pruned tree for L=3 and K=2.

After the last stage 36, the SLIC receiver 34 chooses the best value ofĉ[L] among the K^(L) solutions. The overall solution, i.e., the overallcandidate symbol decision made by the SLIC receiver 34 for the receivedsymbol vector 14 is given by

ĉ=ŝ ^([L])(k ₁ , . . . ,k _(L))=ŝ ^([L−1])(k ₁ , . . . ,k _(L−1))+ĉ^([L])(k ₁ , . . . ,k _(L))  (18)

for the value of the state ŝ^([L−1])(k₁, . . . , k_(L−1)) of thecorresponding Stage L demodulation unit (k₁, . . . , k_(L−1)).

As noted above, the state values may coincide due to SLIC subsetoverlap. Since the same value of the state will produce the same resultsgoing forward, the SLIC receiver 34 is configured in at least oneembodiment to prune duplicates to avoid redundant computations. Pruningstates with constrained tree search offers a further mechanism to avoidcomplexity. In practice, at any stage 36 of the SLIC receiver 34, fewstates have a chance to produce the eventual overall solution ĉ (i.e.,the best one 54 among the candidate symbol vectors 74), based on themetric of their corresponding solutions.

Recall that at stage i, demodulation unit (k₁, . . . , k_(i−1)) producesK outputs ĉ^([i])(k₁, . . . , k_(i−1)). For each value ĉ^([i])(k₁, . . ., k_(i)), the SLIC receiver 34 associates the corresponding metric fromthe demodulation unit (k₁, . . . , k_(i−1)) with the corresponding stateŝ^([i])(k₁, . . . , k_(i)). The SLIC receiver 34 is configured in one ormore embodiments to compare the metrics, and keep a fraction of thestate values with the smallest metrics, and discard the rest. Thisapproach maintains a manageable complexity while allowing the mostpromising state values to survive.

The above approach to pruning represents a breadth-first constrainedsearch algorithm, where candidates for pruning belong to the same levelin the tree. Thus, the SLIC receiver 34 may implement the M-algorithm toperform such pruning. As is known by those skilled in the art, theM-algorithm is a breadth first constrained search whose breadth iscontrolled by a parameter M. At any stage, if there are more than Msurviving state values, the SLIC receiver 34 keeps only the best Mvalues. After the last stage L, the SLIC receiver 34 chooses the bestvalue of ĉ^([L]) among the M solutions. The overall solution is given byĉ=ŝ^([L−1])+ĉ^([L]) as before.

Recall that K is the number of surviving candidates after a search bythe joint detection processor 44 in a given stage 36. However, it can bebeneficial to vary the value of K For instance, the value of K candepend on the stage number—i.e., the serial position of a given stage 36within the overall series of stages 36 in the SLIC receiver 34. Inparticular, the SLIC receiver 34 may use different values of K atsuccessive stages 36 to obtain the same fraction of candidates thatsurvive at any stage, so we can choose K for stage i to be a fraction of(q^([i]))^(N).

Also as noted, the SLIC receiver 34 also may vary the value of thebreadth parameter M across two or more stages 36. In particular, theSLIC receiver 34 may use larger values for M, thus increasing stagenumbers, so that latter stages 36 in the series operate with a largernumber of surviving solutions.

FIGS. 10 and 11 illustrate a non-limiting example of how multiplesurvivor solutions from one or more stages 36 of the SLIC receiver 34lead to multiple solution paths or threads 52 through the series ofstages. FIG. 10 depicts three stages 36-1, 36-2, and 36-3 (L=3).Further, one may assume that there is no pruning and that K equals twoat each of the stages 36. Thus, each stage 36 generates two stage outputsignals 62 for every stage input signal 40 received by the stage 36. Assuch, stage 36-1 generates two stage decision vectors 42, andcorrespondingly outputs two stage output signals 62, each onecorresponding to a respective one of the stage decision vectors 42 and arespective one of the stage input signals 40.

The second stage 36-2 receives the two stage output signals 62 from thefirst stage 36-1 as its stage input signals 40 and generates two stagedecision vectors 42 for each stage input signal 40, meaning that intotal it produces four stage decision vectors 42 and four correspondingstage output signals 62. In turn, the third stage 36-3 receives the fourstage output signals 62 from the second stage 62-2 as its stage inputsignals 40 and generates two stage decisions 42 for each such stageinput signal 40. The third stage 36-3 therefore generates a total ofeight stage decision vectors 42, and the overall SLIC receiver 34produces a total of fourteen stage decision vectors 42, two from thefirst stage 36-1, four from the second stage 36-2, and eight from thethird stage 36-3.

FIG. 11 illustrates that each unique combination of these stage decisionvectors 42 represents a different solution thread 52 running through theseries of stages 36 (and through a particular demodulation unit in eachsuch stage 36). That is, each solution thread 52 represents a differentcombination of stage decision vectors 42 taken across the series ofstages 36, with one stage decision vector 42 taken from each such stage36. The control and processing circuits 50 of the SLIC receiver 34 mayform as many solution threads 52 as there are different combinations ofstage decision vectors 42 across the stages 36. FIG. 11 depicts solutionthreads 52-1 through 52-U, where U≧2.

In general, each solution thread 52 has L members corresponding to the Lstages 36 of the SLIC receiver 34, where each such member is one stagedecision vector 42 from a respective one of the stages 42. As a generalproposition, two or more solution threads 52 may share one or morecommon members, but each solution thread 52 has an overall combinationof output stage decision vectors 42 that is different from the remainingsolution threads 52. The total number of solution threads 52 thereforeis less than or equal to the total number of unique combinations of theindividual stage decision vectors 42 taken across the set of serialstages 36 in the SLIC receiver 34. Each such solution thread 52 will berecognized as representing a different decision path through the SLICreceiver 34, each such path comprising the accumulation of differentdecision states across the stages 36.

As shown in FIG. 12, the decision processor 48 computes a candidatesymbol vector 74 for each solution thread 52, e.g., for threads 52-1,52-2, and so on, the decision processor 48 computes candidate symbolvectors 74-1, 74-2, and so on. Each candidate symbol vector 74 has oneor more corresponding candidate symbol vector (“CSV”) metrics 76, whichindicate how well the candidate symbol vector 74 matches or otherwisecorresponds to the actual received symbol vector 14. The decisionprocessor 48 uses the CSV metrics 76, for example, to select the bestone of the candidate symbol vectors 74 as the best candidate symbolvector 54 shown in FIG. 8, representing an overall demodulation solutionfor the received symbol vector 14.

With the above operations and features in mind, FIG. 13 illustrates oneembodiment of a method of implementing multi-survivor SLIC processing.In at least one embodiment, the method is implemented via appropriateconfiguration of the RX processing circuits 24, including the SLICreceiver 34. The SLIC receiver 34 is implemented in fixed orprogrammable circuitry, or in some combination thereof. In at least oneembodiment, the SLIC receiver 34 is at least partly implemented inbaseband processing circuitry, which may comprise one or more DigitalSignal Processors (DSPs), microprocessors, or other digital processors,and which are configured at least in part according to the execution ofprogram instructions comprising a computer program stored in a memorycircuit or other computer-readable medium included in the circuitrycomprising the wireless communication apparatus 10.

In any case, FIG. 13 illustrates a method 1300 of demodulating areceived symbol vector 14 conveyed in a received signal 32 usingSLIC-based demodulation. In the example illustration, the method 1300includes outputting (Block 1302) at least one stage decision vector 42from each stage 36 in a series of two or more stages 36 of a SLICreceiver 34, and outputting at least two stage decision vectors 42 fromat least one of the stages 36.

Each stage decision vector 42 represents a stage-specific demodulationsolution for the received symbol vector (14) according to astage-specific modulation constellation (effective alphabet Q^([i])).That is, each stage decision vector 42 corresponds to a different jointsolution for the received symbol vector 14, with respect to a particularone of the stage input signals 40 that are applied to the given stage36. In at least one embodiment, each stage 36 generates K stage decisionvectors 42 for each stage input signal 40 that is applied to the stage36. With R stage input signals 40 applied (where R may not be the samefrom stage to stage), there are K×R stage decision vectors 42 generatedat a given stage 36, although with pruning (M<K×R), fewer stage decisionvectors 42 may be output from and used by the given stage 36.

In any case, it will be understood that each stage decision vector 42 isa vector of candidate symbols selected from the stage-specificmodulation constellation(s) Q^([i]) that are used by each i-th stage 36to detect symbols 16 in the received symbol vector. Thus, differentstage decision vectors 42 represent different joint solutions for thesame stage input signal 40, or represent different joint solutions fordifferent ones of the R stage input signals 40.

The method 1300 further includes, for each but the last stage 36,propagating (Block 1304) to the next stage 36 a stage output signal 62for each one of one or more stage decision vectors 42 output by thestage 36, including forming each stage output signal 62 as a function ofa particular one of one or more stage input signals 40 received by thestage 36 and particularly one of the one or more stage decision vectors42 output by the stage 36 for that particular stage input signal 40.

Here it should be noted that stage output signals 62 are interchangeablyreferred to herein as “modified received signals 62,” in keeping withthe fact that each stage output signal 62 is a modified version of aparticular stage input signal 40 that is received by the involved stage36. It should also be noted that the first stage 36-1 in the SLICreceiver 34 receives the received signal 32 as its only stage inputsignal 40, but each remaining stage 36 receives as its stage inputsignal(s) 40 the one or more modified received signals 62 propagatedfrom the prior stage 36.

The method 1300 additionally includes forming (Block 1306) two or moresolution threads 52, as different combinations of stage decision vectors42, where the different combinations are formed by taking one stagedecision vector 42 from each of stages 36 in the SLIC receiver 34, andthe method further includes generating (Block 1308) a candidate symbolvector 74 for at least two of the two or more solution threads 52. Here,the candidate symbol vectors 74, unlike the stage decision vectors 42from which they are formed, are representations of the symbols 16 in thereceived symbol vector 14 in terms of the actual modulationconstellation(s) associated with their transmission.

Thus, the method 1300 includes evaluating (Block 1310) the candidatesymbol vectors 74 to identify a best one of them, denoted as the bestcandidate symbol vector 54, and outputting (Block 1312) this bestcandidate symbol vector (54) as an overall demodulation solution for thereceived symbol vector 14, such as is shown for the example SLICreceiver 34 in FIG. 8.

In one or more embodiments, the set of two or more serial stages 36comprises L stages (36). Thus, forming each of the two or more solutionthreads 52 comprises forming a different combination of L stage decisionvectors 42 taken across the L stages 36.

Further, in one or more of the same or other embodiments, the method1300 includes propagating from a given stage 36 at least two modifiedreceived signals 62 representing different stage decision vectors 42made for the same stage input signal 40 that is input to the given stage36. The method in such embodiments further includes the next stage 36generating at least one stage decision vector 42 for each one of the atleast two modified received signals 62 propagated from the given stage36.

Further, in one or more of the same or other embodiments, the method1300 includes pruning to reduce the number of stage decision vectors 42output by a given stage 36, thereby correspondingly reducing the numberof modified received signals 62 propagated from the given stage 36 tothe next stage 36, in the case that the given stage 36 is not the laststage 36-L. Pruning at the given stage 36 includes generating a firstnumber of stage decision vectors 42 and discarding one or more of thosegenerated stage decision vectors 42, to obtain a reduced number of stagedecision vectors 42 that are output from the given stage 36 for use informing one or more solution threads 52 and that are used by the givenstage 36 to generate the modified received signals 62 propagated to thenext stage 36.

In an example case, the given stage 36 generates K×R stage decisionvectors 42, where R equals the number of stage input signals 40 appliedto the given stage 36 and K equals the number of stage decision vectors42 that are generated per stage input signal 40. A pruning processor 60controls a pruning circuit 46 to retain only M of the K×R stage decisionvectors 42, where M is an integer and is, for active pruning, less thanK×R. Indeed, particularly with fan-out at the latter stages 36, M may besignificantly less than K×R at such stages 36.

As one example, pruning includes evaluating quality metricscorresponding to the generated stage decision vectors 42, e.g., at anygiven stage 36 and with respect to any given one or ones of thecorresponding stage input signals 40, and discarding one or morelower-quality ones of the generated stage decision vectors 42, to obtainthe reduced number of output stage decision vectors 42. Because thequality metrics indicate something about the probabilities that thestage decision vectors 42 are the correct solution for the receivedsymbol vector 14, they may be used to rank a plurality of stage decisionvectors 42, and to select only a subset of the best ones to effectpruning.

Further, assuming that the given stage 36 being pruned is not the firststage 36-1, pruning in one or more embodiments includes discarding anyof the generated stage decision vectors 42 that correspond to duplicatedecision states from the preceding stages 36. In this regard, it will beappreciated that each stage input signal 40 that is input to any givenstage 36 beyond the first stage 36-1 represents a particular decisionstate of the SLIC receiver 34, as accumulated in the correspondingsolution thread 52 running through all prior stages 36.

Further, in one or more of the same or other embodiments, pruningincludes varying the number of stage decision vectors 42 permitted to beoutput from any given stage 36 based on the serial position of the stage(36) within the set of two or more serial stages 36.

More generally, in one or more of the same or other embodiments, themethod 1300 includes varying the number of stage decision vectors 42generated for each stage input signal 40 that is input to each stage 36as a function of the serial position of the stage 36 within the set oftwo or more serial stages 36.

Further, in one or more of the same or other embodiments, the method1300 includes generating the stage decision vectors 42 output by atleast one stage 36 of the SLIC receiver 34 using a constrained treesearch. A constrained tree search searches a reduced space as comparedto a full or unconstrained tree search, and thus offers the advantage ofreduced computational complexity and greater efficiency.

Further, in one or more of the same or other embodiments, the one ormore stage decisions 42 output by each stage 36 are generated using ajoint detection processor 44 that includes one or more demodulationunits and that determines each stage decision vector 42 as a jointsolution representing the received symbol vector 14. This jointsolution, as explained before, comprises a vector of symbols selectedfrom the stage-specific modulation constellation Q^([i]) in use by eachstage 36-i.

Notably, modifications and other embodiments of the disclosedinvention(s) will come to mind to one skilled in the art having thebenefit of the teachings presented in the foregoing descriptions and theassociated drawings. Therefore, it is to be understood that theinvention(s) is/are not to be limited to the specific embodimentsdisclosed and that modifications and other embodiments are intended tobe included within the scope of this disclosure. Although specific termsmay be employed herein, they are used in a generic and descriptive senseonly and not for purposes of limitation.

1. A Serial Localization with Indecision (SLIC) receiver configured todemodulate a received symbol vector in a received signal, said SLICreceiver comprising: a set of two or more serial stages, wherein eachstage is configured to output at least one stage decision vectorrepresenting a stage-specific demodulation solution for the receivedsymbol vector according to a stage-specific modulation constellation,and each stage except for a last stage is configured to propagate to anext stage one or more modified received signals, each modified receivedsignal comprising, if the stage is the first stage, the received signalas modified by the stage to account for a particular one of the at leastone stage decision vector output by the first stage, and otherwisecomprising one of the one or more modified received signals propagatedfrom the prior stage as modified by the stage to account for aparticular one of the at least one stage decision vector output by thestage for said one of the one or more modified received signalspropagated from the prior stage; and wherein at least one stage in theSLIC receiver outputs at least two such stage decision vectors, suchthat SLIC receiver produces at least two solution threads, with eachsuch solution thread comprising a different combination of the stagedecision vectors taken across the set of two or more serial stages; andwherein said SLIC receiver further comprises a decision processor thatis configured to generate two or more candidate symbol vectors fromrespective ones of two or more of the solution threads, each suchcandidate symbol vector representing an overall demodulation solutionfor the received symbol vector, identify a best one of the candidatesymbol vectors, and output the best candidate symbol vector as theoverall demodulation solution.
 2. The SLIC receiver of claim 1, whereinthe set of two or more serial stages comprises L stages, L≧2, andwherein each of the two or more solution threads comprises a differentcombination of L-stage decision vectors taken across the L stages. 3.The SLIC receiver of claim 1, wherein at least one of the stages isconfigured to receive multiple modified received signals propagated fromthe prior stage, referred to as stage input signals with respect to thestage, and to propagate multiple modified received signals to the nextstage, referred to as stage output signals with respect to the stage,and wherein the stage is configured to include in said stage outputsignals at least two different stage output signals corresponding todifferent stage decision vectors determined for the same stage inputsignal.
 4. The SLIC receiver of claim 1, wherein the SLIC receiverfurther includes a pruning processor that is configured to reduce thenumber of stage decision vectors output by one or more of the stages insaid SLIC receiver based on, for a given stage that is configured togenerate multiple stage decision vectors, discarding at least one suchgenerated stage decision vector, to thereby reduce the number of stagedecision vectors that are output by the given stage and, in the casethat the given stage is not the last stage, to correspondingly reducethe number of modified received signals that are propagated from thegiven stage to the next stage.
 5. The SLIC receiver of claim 4, whereinthe pruning processor or an associated pruning circuit is configured toreduce the number of stage decision vectors output by a given stage thatis configured to generate two or more such stage decision vectors, byevaluating quality metrics corresponding to the two or more stagedecision vectors generated by the given stage and discarding one or morelower-quality ones of the stage decision vectors generated by the givenstage, so that the given stage outputs fewer stage decision vectors thanwere generated and, if the given stage is not the last stage,correspondingly propagates modified received signals to the next stageonly for the stage decision vectors that were not discarded.
 6. The SLICreceiver of claim 4, wherein the given stage is not the first stage andwherein the pruning processor or the associated pruning circuit isconfigured to discard any generated stage decision vectors from thegiven stage that correspond to duplicate decision states from thepreceding stages.
 7. The SLIC receiver of claim 4, wherein the pruningprocessor is configured to vary the number of stage decision vectorspermitted to be output from any given stage based on the serial positionof the stage within the set of two or more serial stages.
 8. The SLICreceiver of claim 1, wherein the SLIC receiver is configured to vary thenumber of stage decision vectors that-are generated from each stageinput signal input to any given stage, based on the serial position ofthe stage with the set of two or more serial stages, wherein the firststage receives the received signal as its sole stage input signal, andeach subsequent stage receives as its one or more stage input signalsthe one or more modified received signals output from the prior stage.9. The SLIC receiver of claim 1, wherein at least one stage of the SLICreceiver is configured to use a constrained tree search for determiningthe at least one stage decision vector output by the stage.
 10. The SLICreceiver of claim 1, wherein each stage includes: a set of one or morestage inputs, each configured to receive a stage input signal that isthe received signal if the stage is the first stage and is otherwise oneof the one or more modified received signals output by the prior stage;and a joint detection processor having one or more demodulation unitsand configured to generate at least one stage decision vector for eachstage input signal; and wherein the joint detection processor of atleast one stage is configured to generate more than one stage decisionvector for at least one of the stage input signals.
 11. A method ofdemodulating a received symbol vector conveyed in a received signalusing Serial Localization with Indecision (SLIC) based demodulation,said method implemented in a SLIC receiver and comprising: outputting atleast one stage decision vector from each stage in a series of two ormore stages of the SLIC receiver, each stage decision vectorrepresenting a stage-specific demodulation solution for the receivedsymbol vector according to a stage-specific modulation constellation,including outputting from at least one of said stages two or more stagedecision vectors; for each but the last stage, propagating to the nextstage a modified received signal for each one of the at least one stagedecision vector output by the stage, including forming each saidmodified received signal as a function of a particular one of one ormore stage input signals received by the stage and a particular one ofthe at least one stage decision vector output by the stage for thatparticular stage input signal, wherein the first stage receives thereceived signal as its stage input signal, and wherein each remainingstage receives the one or more modified received signals propagated fromthe prior stage as its one or more stage input signals; forming two ormore solution threads as different combinations of stage decisionvectors, said different combinations formed by taking one stage decisionvector from each of the stages in the SLIC receiver; generating acandidate symbol vector for at least two of the two or more solutionthreads; evaluating the candidate symbol vectors to identify a best oneof the candidate symbol vectors; and outputting the best candidatesymbol vector as an overall demodulation solution for the receivedsymbol vector.
 12. The method of claim 11, wherein the set of two ormore serial stages comprises L stages, L≧2, and wherein forming each ofthe two or more solution threads comprises forming a differentcombination of L stage decision vectors taken across the L stages. 13.The method of claim 11, wherein the method includes propagating from agiven stage at least two modified received signals representingdifferent stage decision vectors made for the same stage input signalthat is input to said given stage, and further includes the next stagegenerating at least one stage decision vector for each one of said atleast two modified received signals propagated from the given stage. 14.The method of claim 11, further comprising pruning to reduce the numberof stage decision vectors output by a given stage, therebycorrespondingly reducing the number of modified received signalspropagated from the given stage to the next stage, in the case that thegiven stage is not the last stage, wherein pruning at the given stageincludes generating a first number of stage decision vectors anddiscarding one or more of those generated stage decision vectors toobtain a reduced number of stage decision vectors that are output fromthe given stage and used by the given stage to generate the modifiedreceived signals propagated to the next stage.
 15. The method of claim14, wherein said pruning includes evaluating quality metricscorresponding to the generated stage decision vectors and discarding oneor more lower-quality ones of the generated stage decision vectors, toobtain the reduced number of output stage decision vectors.
 16. Themethod of claim 14, wherein the given stage is not the first stage andwherein said pruning includes discarding any of the generated stagedecision vectors that correspond to duplicate decision states from thepreceding stages.
 17. The method of claim 14, wherein said pruningincludes varying the number of stage decision vectors permitted to beoutput from any given stage based on the serial position of the stagewithin the set of two or more serial stages.
 18. The method of claim 11,further comprising varying the number of stage decision vectorsgenerated for each stage input signal that is input to each stage as afunction of the serial position of the stage within the set of two ormore serial stages.
 19. The method of claim 11, further comprisinggenerating the at least one stage decision vector output by at least onestage of the SLIC receiver using a constrained tree search.
 20. Themethod of claim 11, wherein the at least one stage decision vectoroutput by each stage are generated using a joint detection processorthat includes one or more demodulation units and that determines eachstage decision vector as a joint solution representing the receivedsymbol vector, said joint solution comprising a vector of symbolsselected from the stage-specific modulation constellation.
 21. Themethod of claim 11, wherein each stage input signal input to any givenstage beyond the first stage represents a particular decision state ofthe SLIC receiver, as accumulated in the corresponding solution threadrunning through all prior stages.